1. Field of the Invention
The present invention relates to electrical heating of subsea pipelines. More particularly the invention relates to making subsea pipelines ready for electrical heating, such that electrical power can be applied to a selected segment of a pipeline after deployment and when heating is needed.
2. Description of Related Art
Offshore hydrocarbon recovery operations are increasingly moving into deeper water and more remote locations. Often satellite wells are completed at the sea floor and are tied to remote platforms or other facilities through extended subsea pipelines. Some of these pipelines extend through water that is thousands of feet deep and where temperatures of the water near the sea floor are in the range of 40xc2x0 F. The hydrocarbon fluids, usually produced along with some water, reach the sea floor at much higher temperatures, characteristic of depths thousands of feet below the sea floor. When the hydrocarbon fluids and any water present begin to cool, phenomena occur that may significantly affect flow of the fluids through the pipelines. Some crude oils become very viscous or deposit paraffin when the temperature of the oil drops, making the oil practically not flowable. Hydrocarbon gas under pressure combines with water at reduced temperatures to form a solid material, called a xe2x80x9chydrate.xe2x80x9d Hydrates can plug pipelines and the plugs are very difficult to remove. In deep water, conventional methods of depressurizing the flow line to remove a hydrate plug may not be effective. Higher pressures in the line and uneven sea floor topography require excessive time and may create more operational problems and be costly in terms of lost production.
The problem of lower temperatures in subsea pipelines has been addressed by placing thermal insulation on the lines, but the length of some pipelines makes thermal insulation alone ineffective. Increased flow rate through the lines also helps to minimize temperature loss of the fluids, but flow rate varies and is limited by other factors. Problems of heat loss from a pipeline increase late in the life of a hydrocarbon reservoir because production rates often decline at that time. Problems become particularly acute when a pipeline must be shut-in for an extended period of time. This may occur, for example, because of work on the wells or on facilities receiving fluids from the pipeline. The cost of thermal insulation alone to prevent excessive cooling of the lines becomes prohibitive under these conditions.
Heating of pipelines by bundling the lines with a separate pipeline that can be heated by circulation of hot fluids has been long practiced in the industry. Also, heating by a variety of electrical methods has been known. Most of the proposals for electrical heating of pipelines have related to pipelines on land, but in recent years industry has investigated a variety of methods for electrical heating of subsea pipelines. (xe2x80x9cDirect Impedance Heating of Deepwater Flowlines,xe2x80x9d OTC 11037, May, 1999.) Previously developed apparatus included xe2x80x9cCombipipe,xe2x80x9d which employs electrical conductors in the insulation layer of a pipe, (xe2x80x9cHeating of Pipelines, and Power Supply to Subsea Electrical Equipment,xe2x80x9d DOT, 1995) and heat tracing, which employs a conductor inside a heat tube in the vicinity of the pipeline (xe2x80x9cA New Method for Heat Tracing Long Pipelines,xe2x80x9d ASME 74-Pet-35, 1974).
Two configurations for electrical heating have been particularly considered in recent years. In one configuration, a single flowline is electrically insulated and current flows along the flowline. This is called the xe2x80x9cSHIPxe2x80x9d system (Single Heated Insulated Pipe). Two SHIP systems have been considered: the fully insulated system, requiring complete electrical insulation of the flowline from the seawater, and the earthed-current system, requiring electrical connection with the seawater through anodes or other means. For both systems, current is passed through the flowline pipe. A fully insulated method of electrically heating a pipeline is disclosed in U.S. Pat. No. 6,049,657. In this method, an electrically insulated coating covers a single pipeline carrying fluids from a well. An alternating current is fed to one end of the pipeline through a first insulating joint near the source of electrical current and the current is grounded to seawater at the opposite end of the pipe through a second insulating joint.
In a second configuration for electrical heating, a pipe-in-pipe subsea pipeline is provided by which a flow line for transporting well fluids is the inner pipe and it is surrounded concentrically by and electrically insulated from an electrically conductive outer pipe until the two pipes are electrically connected at one end. Voltage may be applied between the inner and outer pipes at the opposite end and electrical current flows along the exterior surface of the inner pipe and along the interior surface of the outer pipe. This pipe-in-pipe method of heating is disclosed, for example, in Ser. No. 08/921,737, filed Aug. 11, 1999, which is commonly assigned and hereby incorporated by reference herein. A center-fed pipe-in-pipe configuration is disclosed in the commonly assigned application titled xe2x80x9cElectrical Heating of Pipelines with Pipe-in-Pipe and Mid-Line Connector,xe2x80x9d filed concurrently herewith and hereby incorporated by reference herein.
In all the configurations for electrical heating, it will often not be necessary to supply power to the pipeline continuously. In fact, heating may not be needed until years after a pipeline is deployed. For example, heating may only be needed with the production rate from an oil or gas field has declined, such that the fluids cool more in moving through a pipeline. A temporary interruption in flow through a pipeline may cause the need for heating, but after a plug has been removed there will be no need for heating. Also, only segments of the pipeline may require heating at any timexe2x80x94where plugging has occurred or where it is considered more likely. Installation of electrical cables or other facilities for heating a subsea pipeline requires significant capital expenditures. It will be advantageous to delay as many of these expenditures as long as possible. Therefore, there is a need to install subsea pipelines that can be heated only when the heating is required for optimum operation of the pipeline system. Such pipelines will be referred to as xe2x80x9celectrically ready.xe2x80x9d
Toward providing these and other advantages, apparatus and method are provided for enhancing the flow of fluids through a subsea pipeline by heating a segment of the pipeline using portable or fixed electrical power generation equipment that is connected after the pipeline is deployed to the seafloor. The apparatus and method may be applied to the pipe-in-pipe configuration, the Single Heated Insulated Pipe (SHIP) configuration or any other configurations. The power generation equipment may be a conventional electrical generator mounted on a ship or a fixed structure. Preferably, alternating current is used, but direct current may also be used. Multiple segments of the pipeline may be heated, either contiguous segments or discontinuous segments, or the entire pipeline may be considered a segment.
In one embodiment, a mid-line electrical connector is installed with the pipe-in-pipe configuration and a wet-mateable connector is attached to the mid-line connector. The wet-mateable connector may be attached directly to the mid-line connector or it may be attached through a cable. The cable may be buoyed so that is more easily accessed by a Remotely Operated Vehicle (ROV). An ROV may be used at a later time to connect electrical power to the mid-line connector.
In another embodiment a wet-mateable connector is connected to one or more of the electrical connectors used to pass current through a segment of a Single Heated Insulated Pipe. The wet-mateable connector may be attached directly to one or more of the connectors or it may be attached through a cable. The cable may be buoyed and connected to electrical power at a later time. There may be a midline connector in the segment to be heated and it likewise may be accessible through a wet-mateable connector. The mid-line connector may be formed on the seafloor from a connector penetrating the insulation coating on the pipeline.
An electrically ready heated pipe is heated by connecting a source of power to the electrical conductors used in heating after the pipeline is deployed on the seafloor. For any configuration, the source of power may be lowered to the vicinity of a wet-mateable connector and connected, which may employ an ROV. To minimize power loss in an umbilical used to supply electrical power, the voltage may be stepped up at the source of power and stepped back down in the vicinity of the connector to the pipeline. The power factor of the power supplied may be improved to increase efficiency of heating.
In another embodiment a toroidal transformer is used to apply power. In another embodiment sea water electrodes are used to complete an electrical circuit through a segment of pipeline to be heated. In yet another embodiment, power is supplied to a riser along with a segment of the pipeline on the sea floor. Multiple segments may be heated simultaneously or in sequence. Selection of segments to be heated at any time may be made from measurements of pressure or other physical variables indicating where heating is needed.
In yet another embodiment, a toroidal transformer is used to extract power from a selected location along the segment being heated. A wet-mateable connector may be used to connect with the wire of the toroidal transformer. The power extracted may be used to operate a device, heat another short segment of pipeline or for other purposes.